Many medicinal substances are administered by intravenous infusion. The toxicity, the requirements as to regularity in the timing of the doses and the physical condition of the patient mean that it is desirable, even essential, to monitor the flow rate of an intravenous infusion.
Infusion by gravity, with manual adjustment of the flow rate by progressive squeezing of the infusion tubing and visual monitoring of the flow rate using a drip cylinder is still the most commonly used infusion method today. It is obvious that the operation of adjusting the flow rate is far from being precise. The nurse must first calculate, in terms of the desired duration of the infusion and the number of drops per unit volume (which depends on the liquid infused and the drip cylinder being used), the time interval between two drops. Then she has to adjust the flow rate by squeezing the infusion tubing while timing the drops in the drip cylinder using a watch. During the whole period of the infusion, she must regularly return to carry out a visual check of the remaining volume and the flow rate of the drops in the drip cylinder, in order to decide subjectively whether the progress of the infusion is roughly acceptable. If she suspects too slow or too fast a rate, she adjusts the flow rate largely by using her own intuition rather than by a recalculation or by retiming the drops.
If there is a desire for greater precision in the adjustment of the flow rate and for assurance that the infusion flow rate is adhered to for the whole duration of the infusion, infusion devices with dosing pumps (also called peristaltic pumps) designed to deliver an almost constant flow rate must be available. However, such devices unfortunately have drawbacks limiting their use. First, the consumption of electrical power by dosing pumps is far from being negligible. When the power is supplied by batteries, the autonomy of these devices is restricted as a result. Their appreciable weight and bulk mean that these devices can rarely be used for other than bedridden patients. Intravenous infusion devices with dosing pumps also entail the use of special tubing (flexible tubes made of silicone). The cost of such special tubing is much greater than that of standard tubing, and its supply complicates the handling of consumable equipment. The use of a dosing pump also means that the delivery pressure has to be monitored. This pressure depends on the back pressure at the point of injection and is therefore a parameter very sensitive to the smallest anomalies. It should also be noted that the very measurement of the delivery pressure is problematic. In effect, because the tubing must remain sterile, the pressure sensors can only be outside the tubing. Now, it is not obvious how to measure an excess pressure through plastic tubing.
Intravenous infusion devices with dosing pumps on the market today are complex automatic items equipped with sophisticated control and adjustment systems, which are often the source of untimely alarms. In these sophisticated control and adjustment systems, it is possible to incorporate a device for controlling the infusion flow rate. Such a flow rate control device comprises an electro-optical sensor of the optical barrier type, which is combined with a drip cylinder to detect the presence or absence of a drop. The signal from the sensor is processed in a counting and timing unit. An ALU (arithmetic and logic unit) monitors the flow rate of the drops in order to trigger an acoustic and/or visual alarm and/or to intervene in the system for adjusting the dosing pump when the flow rate of drops shows a positive or negative deviation from a preset value.
Such a flow rate monitoring device is described, for example, in the document EP-A-0441323. In order to increase the sensitivity of the detection, this document proposes to replace the photodiode by a kind of miniature camera for detecting an image of the drop. It is obvious that such a method does not reduce either the price of the device, or its bulk, or its electrical power consumption.
It would also be of interest to use devices for controlling the infusion flow rate to monitor infusions by gravity. Now, although the price of the flow rate control device is only a minor factor in the case of complex and expensive automatic infusion controllers, this is not so if there is a desire for the systematic use of an autonomous flow rate monitoring device for monitoring infusion by gravity. Such a flow rate monitoring apparatus should be cheap (in view of the large number of appliances required in a hospital), be very compact and consume little power (so as not to restrict the mobility of a patient who is not bedridden), be very convenient to use (so as to guarantee its acceptance by medical staff) and guarantee a high degree of safety in use.
This complex problem, which at first sight requires the solution of completely contradictory problems, finds a simple solution in a device according to the invention.